atill pretty ,meaningless as most people wouldnt know the body weight of their horse if it fell on them. Weight tapes are a joke, and how many people put their horses on an actual scale ? I have, but as muscles, fat and hydration levels change those numbers change also. A 920lb horse can loose 40 to 50 lbs of water pretty quickly.
Breed, confirmation, condition, age, temperature, terrain, all have a much greater influence on load ability than some arbitrary pulled out of thin air percent.
BTW Gen George McClellan, an admitted horse lover and man hater did a study using data collected in the summer in the soutwest, that found Calvary horses, could carry 20 % all day every day, and only be fed at night and last for months with out breaking down. Of course again, how did he weigh the horses ? and when did he weigh them morning or after being rode all day. Anyway he published an Army cavalry manual using those numbers and they have been getting repeated as the gospel ever sense.
Keep in mind this self proclaimed "expert" in cavalry consistently had his cavalry embarrassed by the poor unfortunate unedumacated southern cavalry.